Procedures for the quality assurance of cans normally include optical inspection of the surface of the cans, not only to detect scratches or dents but also to detect possible defects of the decorative enamel coating previously applied to the surface of the cans by one or more rollers and subsequently fixed in an oven.
Optical inspection is usually conducted along the conveying section of a conveyor belt which transports the cans one after the other in a row. A lighting system is directed towards the conveying section so as to illuminate each can as it moves in the field of vision of a camera. Lastly, the image captured by the camera is compared with a reference image to determine whether the can inspected is to be accepted or rejected.
Examples of optical inspection apparatus are provided by patent documents EP1477794A1, EP0961113A1, EP1985997A1, US2011/050884A1, JP2002/267611A, U.S. Pat. No. 7,911,602B2, US2003/184740A1.
One drawback of prior art inspection systems is that the image of the can captured by the camera is undesirably affected by reflected light from the adjacent cans upstream and downstream of the can being inspected.
This drawback is due largely to the fact that the undecorated parts of the cans tend to reflect the incident light like a mirror.
In one possible solution to this drawback, the cans are spaced apart by a length such as to reduce or eliminate the effect of reflected light. This solution, however, means slowing down the optical inspection process, which in turn depends on the maximum speed at which the conveyor belt can run.